[PATCH] coredump: shutdown current process first

This patch optimizes zap_threads() for the case when there are no ->mm
users except the current's thread group. In that case we can avoid
'for_each_process()' loop.

It also adds a useful invariant: SIGNAL_GROUP_EXIT (if checked under
->siglock) always implies that all threads (except may be current) have
pending SIGKILL.

Signed-off-by: Oleg Nesterov <[e-mail]>
Cc: Roland McGrath <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

fs/exec.c

···
 static void zap_process(struct task_struct *start)
 {
 	struct task_struct *t;
-	unsigned long flags;
 
-	/*
-	 * start->sighand can't disappear, but may be
-	 * changed by de_thread()
-	 */
-	lock_task_sighand(start, &flags);
 	start->signal->flags = SIGNAL_GROUP_EXIT;
 	start->signal->group_stop_count = 0;
 
···
 			signal_wake_up(t, 1);
 		}
 	} while ((t = next_thread(t)) != start);
-
-	unlock_task_sighand(start, &flags);
 }
 
 static inline int zap_threads(struct task_struct *tsk, struct mm_struct *mm,
 				int exit_code)
 {
 	struct task_struct *g, *p;
+	unsigned long flags;
 	int err = -EAGAIN;
 
 	spin_lock_irq(&tsk->sighand->siglock);
 	if (!(tsk->signal->flags & SIGNAL_GROUP_EXIT)) {
-		tsk->signal->flags = SIGNAL_GROUP_EXIT;
 		tsk->signal->group_exit_code = exit_code;
-		tsk->signal->group_stop_count = 0;
+		zap_process(tsk);
 		err = 0;
 	}
 	spin_unlock_irq(&tsk->sighand->siglock);
 	if (err)
 		return err;
 
+	if (atomic_read(&mm->mm_users) == mm->core_waiters + 1)
+		goto done;
+
 	rcu_read_lock();
 	for_each_process(g) {
+		if (g == tsk->group_leader)
+			continue;
+
 		p = g;
 		do {
 			if (p->mm) {
-				if (p->mm == mm)
+				if (p->mm == mm) {
+					/*
+					 * p->sighand can't disappear, but
+					 * may be changed by de_thread()
+					 */
+					lock_task_sighand(p, &flags);
 					zap_process(p);
+					unlock_task_sighand(p, &flags);
+				}
 				break;
 			}
 		} while ((p = next_thread(p)) != g);
 	}
 	rcu_read_unlock();
-
+done:
 	return mm->core_waiters;
 }